The invention relates to a display device comprising an electro-optical display material between a first substrate provided with row electrodes, and a second substrate provided with column electrodes, in which overlapping parts of the row and column electrodes define picture elements, the device further comprising driving means for providing the row electrodes with selection pulses having a selection pulse-width and a selection pulse voltage and for providing the column electrodes with data pulses.
Such display devices are commonly known as passive displays and are used in e.g. mobile phones and portable computers.
A general way of driving these types of displays is known as multiplexing: the RMS-voltage across a picture element, or pixel, determines the light transmission. In passive displays each column electrode as well as each row electrode is common to several pixels. Generally, time-multiplexing is used, in which (subsequent) rows of pixels are selected subsequently during a row selection period, while data-voltages are simultaneously supplied to the column electrodes, dependent on the information to be written. After all rows have been selected (one frame time), this is repeated.
To obtain grey-values (or colours if a birefringent liquid crystal effect is used, such as ECB or STN), pulses of different pulse-width are used for different grey-values (or colours), which implies switching of the data pulse during each selection period at least once, if data is available. For each switching action, the pixel capacitance has to be loaded or reloaded, which is a major source of current (power) consumption in LCD driving circuitry.
It is an object of the present invention to overcome the above mentioned problems at least partly.
A display device according to the invention is therefore characterized in that the device comprises means for diminishing during operation the selection pulse widths within a frame time based on an extreme pulse width of the data pulses to be applied to the column electrodes within a frame time and further driving means for applying, in operation, correction voltages across the picture elements during said frame time.
The invention is based on the recognition that said diminishing of pulse widths renders the above-mentioned switching superfluous for the pulses related to the lightest and/or darkest colour or grey-value within a frame. To guarantee the right colour (grey-value), all pixels are given an extra voltage simultaneously before or after a frame. Because this correction depends on the RMS-value to be corrected, either a voltage correction during the full frame time can be applied, or a pulse width correction can be applied.
A first embodiment is characterized in that the means for diminishing the selection pulse widths comprise means to diminish the selection pulse widths by the minimum pulse width of a data pulse within the frame time. Preferably, the further driving means comprise means for providing row electrodes simultaneously with a pulse having the minimum data pulse width within the frame time during the remainder of the frame time and means for providing the column electrodes simultaneously with a data-pulse. This guarantees optimum contrast.
A further embodiment is characterized in that the means for diminishing the selection pulse widths comprise means to diminish the selection pulse widths by the minimum difference between the selection pulse width and the width of a data pulse within the frame time. Preferably, the further driving means then comprise, for example, means for providing row electrodes simultaneously with a pulse having a pulse width equal to the minimum difference between the maximum pulse width of a selection pulse and the width of a data pulse within the frame time during the remainder of the frame time and means for providing the column electrodes simultaneously with a non-data pulse. This guarantees optimum contrast again.
These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter.